1. Field of the Invention
The present invention relates to a spark plug used for providing ignition of an internal combustion engine.
2. Description of the Related Art
Various spark plugs used for providing ignition of an internal combustion engine, such as an automotive engine, have been proposed in which a columnar noble metal tip is disposed not only on the tip end of a center electrode, but also on the tip end of a ground electrode. When a center electrode and a ground electrode constituting a first discharge gap become worn as result of spark discharge, durability is reduced. Therefore, durability is maintained by forming a noble metal tip at a position of the ground electrode corresponding to the first discharge gap.
Recently, there has been an increasing demand for enhancing the performance of an engine, and thus there is a need to further improve the ignitability of a spark plug. In order to improve ignitability, a configuration where a columnar noble metal tip is thinned is effective for the following reason. A flame kernel produced in a first discharge gap is caused to grow by swirling or the like. During the growing process, the noble metal tip makes contact with the flame kernel to impede its growth (hereinafter, this is also referred to as a flame quenching effect). When the noble metal tip is thick, the flame kernel easily makes contact with the noble metal tip, and hence is readily affected by the flame quenching effect. Therefore, a structure is often used in which the noble metal tip is thinned to suppress influence of the flame quenching effect due to contact of a flame kernel with the noble metal tip, thereby expediting growth of the flame kernel.
Another configuration where the distance in the axial direction is increased, between a face of the noble metal tip opposing the center electrode and the inner side face of the ground electrode (specifically, the body of the ground electrode) on the side of the center electrode (hereinafter, this distance is also referred to as a protrusion amount), is also effective for the following reason. When the protrusion amount of the noble metal tip is small, a flame kernel easily makes contact with the ground electrode body at an early stage of the growing process of the flame kernel, and hence the flame quenching effect readily occurs. Therefore, a structure in which the protrusion amount is increased so as to expedite the growth of a flame kernel is often employed (see JP-A-2001-345162).
The present inventors have found that the discharge voltage fluctuates in the spark plug of JP-A-2001-345162. In the case where such fluctuation occurs, when the discharge voltage is high, a flame kernel is hardly formed in a first discharge gap. Hence, ignitability is impaired, thereby resulting in a possibility of misfiring. In the case where carbon or the like adheres to the surface of an insulator to produce a so-called “fouling” state, when the discharge voltage is high, there is a possibility that a spark discharge is not produced between a center electrode and a ground electrode, but rather occurs between the center electrode and a metal shell while creeping along the surface of the insulator.